Sound recording and reproduction



Jan. 20, 1953 G. BERGsoN soUND RECORDING AND REPRODUCTION Filed Qmy 27, 1949 Fia, 5

AMX.

MAX.

Patented Jan. 20, 1953 SOUND RECORDING AND REPRODUCTION Gustav Bergson, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application July 27, 194.9, Serial No. 107,034

7 Claims. 1

This invention relates to sound recording and reproduction, and particularly to the recording of sound on film, either photographically or mechanically.

Photographic recording on emulsions carried by motion picture iilm is well-known, such recordings being of two major types, namely, variable area recording, whereby the width of a transparent area varies with the amplitude of the signal, and the variable density type of recording, whereby the :density of the record varies with the amplitude of-the signal. The present invention is directed to a recording system whereby two, narrow, transparent lines vary in separation in accordance with the amplitude of the signal, these lines being made by exposing and processing the iilm or utilizing an opaque lm and mechanically cutting the narrow lines therein, so that light may pass through them.

The invention also utilizes the eiect of interference between two monochromatic light beams in diierent and Varying lengths of transmission paths, or the color pattern produced by light interference in cooperation with a color selective photoelectric cell, such as is disclosed and claimed in U. S. Patent No. 2,423,254 of July l, 1947. By the use of these methods and systems of recording and reproduction, the effect of scratches or dirt is reduced to a minium, since the ratio of transparent area to opaque area is very small and remains constant. Also, since the centers of the spots of light dominate the interference operation, distortion eiiects due to light spill-over are minimized. Furthermore, the effect of film graininess may be substantially eliminated.

The principal object of the invention, therefore, is to facilitate the recording and reproduction of sound.

Another object of the invention is to provide an improved system for recording and/or reproducing sound. l

A further object of the invention is to provide an improved recording system wherein the ratio between the opaque area and transparent area of the sound track area is large and remains constant at all signal amplitudes.

A still further object of the invention is to provide an improved sound reproducing system wherein noises caused by scratches, dirt, light spill-over, and lm graininess are reduced to a minimum.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization andthe mode of its operation will be better understood by referring to the following description, read in conjunction with the accompanying drawings, forming a part hereof, in which:

Fig. 1 is a diagrammatic view of a recording system embodying the invention.

Fig. 2 is a detailed view of a slit and light beam showing a modiiication of the invention.

Fig. 3 is a ldiagrammatic View of a sound reproducing system embodying the invention.

Fig. 4 is a diagram illustrating the detecting action of the reproducing; system of the invention, and

Fig. 5 is a graph showing the characteristic between light beam separation and detection response in accordance with the invention.

Referring now to the drawings, in which the same numerals identify like elements, light from a light source 5 is collected by a lens 6 and projected upon an aperture plate 8 having two, small, equal-sized apertures 9 and l0 therein. The light emerging from aperture 9 is received on a xed mirror I2, from which it is reflected through an objective lens I3 to form straight trace i5 on a lm I6, which may or may not contain picture frames Il. Light emerging from aperture l0 is received on a mirror 20, which is oscillated perpendicularly to the motion of the iilm by galvanometer 30 in accordance with the amplitude and frequency of the signal being recorded. The light from the mirror 20 is projected by projection lens I 3 to form the variable rtrace 22, shown on the sound track portion of the iilm IBL This system is simple in' operation, in that, as the film is advanced past the two beams, the trace I5 will remain a constant distance from'the edge of the lm, while the trace 22 will vary in separation from trace l5 in accordance with the amplitude and frequency of the signal being recorded. When the lrn I6 is developed, the two traces will form opaque lines, which, when printed, will be transparent,.the remaining portion of the sound track area being opaque. Thus, regardless of the amplitude of the signal or its frequency, the ratio between the opaque area and transparent area will Vbe large and remain the same. In the case of present-day variable area and variable density recording, the ratio of transparent area to opaque area increases with signal amplitude.

Referring now to Fig, 2, if the aperture plate 8 had a V-shaped slot, it would form a V-shaped light beam, as shown by the broken lines 25, this beam falling on the slit mask 26 with a slit 2l therein. Now, when the light beam 25 is vibrated perpendicularly to the slit 21 in accordance with the amplitude and frequency of the signal being recorded, two traces will be formed in the sound track area, which will approach and recede from one another in proportion to the amplitude of the signal. This modification requires only one mirror 2D and galvanometer 30, which vibrate in the direction of motion of the film instead of perpendicularly thereto. Also, the track formed would have two varying traces and be in the form of a bi-lateral record, vinstead of a uni-lateral record with a fixed trace and only one variable trace.

Referring now to Fig. 3, illustrating the reproducing system, light from a xed source 30 is collected by a lens 3I and impressedon a slit mask 32 having a slit 33 therein. The narrow line of light emerging through the slit 33 is impressed on the sound track area of the Y`lm I6 andthe two beams or spots of light, through 4the traces I5 and 22, are projected py lenses 35 through a light iilter 36, the slit of a mask 31, to a photoelectric cell '39. The photoelectric cell is connect'ed through an ampliiier `III to a kspeaker 42. By the use of lter 36, a monochromatic light, such as green, having a wavelength of approximately 5000 angstrom units, will be impressed on the cell 39.

'Ihe manner of detecting the light is based on the interference pattern between the beam passing through the trace I5 and the beam passing through the trace 22 at different Vdistances lof separation. As shown in Fig. 4, the fixed trace I5 and variable trace 22 are shown in one position of separation.

For a general 'determination of the photoelectric cell position, let it be assumed that the traces I5 and 22 are separated from each other in steps of small, equal increments. Insuch oase,

dk sin QzXk (1) and, to a high degree of approximation,

Adk sin QIAXk (2) where dk is the separation of the traces I5 and 22 at zero signal, Q is the angle of the first maximum with respect to thecenter line between the two trace positions at zero signal, and )ik is the wavelengthof the light illuminating the cell 39, and which has this first maximum at angle Q at Zero signal. Therefore, lfor a linear quantized recording system (that is, Ade `is proportionfal -to signal intensity), the light 'band separation must be uniform in wavelength andthe absorption of the lter must be a linear function of the wavelength. Thus, the over-all product of band intensity, photocell sensitivity, and filter Vabsorption is the factor which is a linear functionof the wavelength. For green light, as mentioned above, the first maximum is at b, approximately ten degrees off the above-mentioned centerline, Which is the normal tothe film'at the base line, as shown by dotted line a in Fig. 4. The preceding minimum is approximately ve degrees off the normal, as shown by dotted line c. The cell 39 is now placed mid-way between these positions b and c; thatis, at point d in Fig. 4. Since a substantial linear relationship Aexists. between maximum and minimum points, the 'cell response with light trace separation will'follow the curve e in Fig. 5. p

When non-monochromatic light is used, such as la wavelength band between 4000 and v6000 angstrom units, the cell position is determined by the equation:

where'Bisthe'band intensity at wavelength A, S

iilm.

is the photocell sensitivity at wavelength A, and A is the filter absorption at wavelength A. Thus, increased beam separation causes longer Wavelengths to illuminate the photocell, and high level sounds correspond to longer wavelengths as illustrated in the above mentioned patent. Thus, the output of photocell 39 is dependent upon the ratio or differenc'ein the length ofthe light transmission paths from the traces as they Vvary in separation.

`The above system, therefore, provides a sound recording jand reproducing system which has a minimum of distortion due to scratches and dirt onlth'e record carrier or light spill-over at the If the nlm is opaque, a fixed stylus could cut trace I5 and a stylus varying with the amplitude'and frequency of a signal could cut trace 22,

4thus providing a mechanical recorder, the light beams passing through the traces being reproduciblevin the same manner as photographically produced traces.

I claim:

l. A sound recording system comprising means for producing a plurality of light beams, a film having a light sensitive emulsion thereon, and means for projecting said beams to said emulsion, said last mentioned means including means for varying the separation of said beams on 'said emulsion in accordance with the amplitude and frequency of a signal being recorded, and said first mentioned means including a mask with a V-shaped aperture therein and a mask with a straight slit therein on which light from said aperture is projected to provide on a print two transparent traces narrow with respect to the opaque portion of said emulsion.

2. A 'sound reproducing system comprising a film having thereon a pair of narrow light traces varying in separation in accordance with the amplitude and frequency of a signal, said traces being transparent and of a small width compared with the opaque portion of said film, means for forming a narrow light beam, means for projecting said narrow beam on said film, a photocell, a filter between said iilm and said photocell to predetermine the wavelengths of the light impressed on said photocell, and means for positioning said photocell in a position so that the light paths of the beams emerging from said film have different lengths varying with the separation between said beams and coincident at some point between said photocell and said filter.

3. A sound reproducing system for reproducing sound from a pair of light beams whose separation varies in accordance with the amplitude and frequency of a signal comprising a film having a pair of transparent traces thereon varying in separation, the transparent area of said traces being small with respect to the opaque area of said film, means for obtaining light beams corresponding to the position of said traces transversely of said film at any particular instant, a photocell, and optical means for projecting on said photocell said light beams at an angle to a line perpendicular to said iilm and along dierent length paths, said beams coinciding at some point along their paths, the ratio of the variation in length of said paths determining the output of said photocell.

4. A sound reproducing system for reproducing sound from a pair of light beams whose separation varies in accordance with the amplitude and frequency 'of a signal comprising a iilm having a pair of traces thereon varying in separation, the transparent portion of said traces being small in comparison with the opaque portion of said lm, means for obtaining light beams corresponding to the position of said traces at any particular instant, a photocell, and means for projecting on said photocell said light beams along different length paths. said paths coinciding at some point along their lengths, the ratio of the variation in lengths of said paths determining the output of said photocell, said lastmentioned means including a light filter and optical means positioned at an angle to a line perpendicular to said lm.

5. A sound reproducing system for reproducing sound from a pair of light beams Whose separation varies in accordance with the amplitude and frequency of a signal comprising a film having a pair of traces thereon varying in separation, the transparent area of said traces being small in comparison with the opaque area of said film, means for obtaining light beams corresponding to the position of said traces at any particular instant, a photocell, and means for projecting on said photocell said light beams along different length paths, said paths coinciding at some point along their lengths, the ratio of the variation in lengths of said paths determining the output of said photocell, said last mentioned means including a light-filter transmitting light having a wavelength of approximately 5000 angstrom units.

6. A system for detecting a sound record consisting of a pair of longitudinal traces varying in separation in accordance with the amplitude and frequency of a signal, said traces being transparent and the area of said traces being small compared with the opaque area of said sound record, cmprising means for generating a pair of beams varying in separation at said record in accordance with the separation of said traces. means for projecting said beams to a common point along different length paths, the diierence between the lengths of said paths being determined by the separation of said beams at, said record, said beams coinciding at some point along their diierent length paths, and means for detecting the interference between said beams at said common point.

'7. A system for detecting a sound record having a pair of light passing traces which vary in separation in accordance with the frequency and amplitude of a sound signal comprising means for producing light sources at points on said traces transversely of said record, projecting means for substantially monochromatic light from said sources as light beams along different length paths to a common point, and means for detecting the varying light intensity at said point produced by the interference of light in said beams.

GUSTAV BERGSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,840,791 Page Jan. l2, 1932 

